1. Field of the Invention
This invention relates to an improvement of a voltage controlled oscillator whose oscillation frequency band is changeable.
2. Description of the Prior Art
FIG. 3 shows schematically a portion of a transmitter-receiver for personal radio communication, in which an antenna 1 is connected through an antenna switch 2 to a high-frequency amplifier 3 and transmission power amplifier 5. On the output side of the high-frequency amplifier 3 a first mixer 4 is connected through a transmit-receive switch 6 to a voltage controlled oscillator (hereinafter abbreviated as VCO) 8.
The VCO 8 used in the transmitter-receiver for personal radio communication must operate as a carrier oscillator and modulator at the time of transmission and as a local oscillator for frequency conversion at the time of reception, so it has a configuration by which it supplies an oscillation signal in frequency bands differing between the transmission time and reception time. For this type of VCO, recently, a distributed parameter line of 1/4 wavelength resonance mode is used as a resonance circuit, because it makes the circuit configuration simple and provide good grounding for the resonance circuit.
More clearly, the VCO 8 has a known circuit configuration as shown in FIG. 4 which includes a modified Colpitts oscillator, wherein the collector of a transistor 9 forms an amplifying circuit, and a coaxial dielectric line 11 of 1/4 wavelength resonance mode and a series circuit consisting of a coupling condenser 12 and variable capacitance diode 13 are provided in parallel and connected with transistor 9 through a Clapp condenser 10. The dielectric line 11 and variable capacitance diode 13 are grounded respectively. The emitter of the transistor 9 is connected through a DC-blocking/coupling condenser 14 to an output terminal 15 for outputting the oscillation signal. Across the emitter-collector a feedback condenser 16 is connected, and across the base-emitter a feedback condenser 18 is connected. To change the frequency band of the oscillation signal between time transmission time and reception, a series circuit consisting of a condenser 23 and switching diode 24 is connected in parallel across the dielectric line 11, and the connection point between the condenser 23 and diode 24 is connected to a transmit-receive switching terminal 25 through a resistor 7. In the drawing, 17 is a grounding condenser, and 19 through 22 are bias resistors.
The operation of the VCO 8 when the transmitter-receiver is in the reception state will now be described.
At the time of reception a voltage lower than the ground potential is applied to the transmit-receive switching terminal 25, so that the switching diode 24 is kept in the non-conducting state and the capacitance Ca of the condenser 23 does not contribute to a parameter which determines the resonance frequency. Thus, a series-parallel circuit formed by the line 11, coupling condenser 12, variable capacitance diode 13, and Clapp condenser 10 becomes inductive as a whole, and by the above series-parallel circuit and feedback condensers 16 and 18 connected in parallel therewith a parallel resonance circuit is formed. Accordingly, as a voltage is applied to a power terminal 26 connected to the base and collector of the transistor 9, the circuit starts to oscillate at the resonance frequency of the above parallel resonance circuit, and the resultant oscillation signal is amplified by the transistor 9 and output through the output terminal 15. This oscillation signal has a frequency (about 961-963 MHz) which is the sum of a frequency of about 903-905 MHz and a first intermediate frequency (about 58 MHz), and is supplied through the switch 6 to the first mixer 4 (see FIG. 3).
On the other hand, a signal (about 903-905 MHz) received by the antenna 1 is, as shown in FIG. 3, amplified by the high-frequency amplifier 3 after passing through the antenna switch 2, and supplied to the first mixer 4. Then, because the received signal is mixed with the oscillation signal given from the VCO 8 in the first mixer 4, the first intermediate frequency of about 58 MHz is output from the mixer 4.
Following the above process, as shown in FIG. 3, a control voltage corresponding to a desired channel frequency is given from a PLL (phase-locked loop) circuit 7 to a control terminal 27 of the VCO 8, and this control voltage is applied to the variable capacitance diode 13, as shown in FIG. 4, to vary the capacitance thereof, so that a capacitance Cc resulting from the combination with the coupling condenser 12 is changed and the frequency of the oscillation signal is set to the desired channel frequency. As a result, it is possible to select the desired channel.
On the contrary, at the time of transmission a positive voltage is applied to the transmit-receive switching terminal 25, so that the switching diode 24 becomes conductive and the capacitance Ca of the condenser 23 is included in the resonance circuit. Thus, the whole resonance capacitance is increased and becomes the sum of Cc+Ct+Ca and capacitances of the feedback condensers 16 and 18. Accordingly, the whole resonance capacitance and dielectric line 11 form the parallel resonance circuit and the capacitance Cc varies in response to the control voltage given from the PLL circuit 7, so that the oscillation signal having the desired channel frequency (within the range of about 903-905 MHz) is obtained as a carrier. This oscillation signal is, after being modulated by a modulation signal supplied to a modulation terminal 28 (see FIG. 3) of the VCO 8, applied through the switch 6 to the transmission power amplifier 5 in which the signal is amplified up to a given power level, and supplied through the antenna switch 2 to the antenna 1, whereby the signal is radiated into the air.
As is apparent from the foregoing description, according to the conventional VCO 8, because the condenser 23 is connected in parallel with the dielectric line 11, the capacitive component Ca of the condenser 23 can be added to the resonance capacitance by making conductive the switching diode 24; thus the frequency band of the oscillation signal can be changed in correspondence to the transmission state and reception state.
The conventional oscillator, however, has the drawback that the sensitivity of the VCO varies in response to switching of the oscillation frequency band if the condenser 23 is connected in parallel with the dielectric line 11 and its capacitance Ca is added to the resonance capacitance.
More clearly, by denoting the minimum value of the capacitance Cc+Ct of the variable capacitance diode 13 by C and an inductance given upon viewing the network of FIG. 4 at point a in the direction opposite to the arrow by L under the condition that the switching diode 24 is non-conducting, the resonance frequency f is represented by the following equation: ##EQU1##
Denoting the maximum value of the capacitance of the variable capacitance diode 13 by C+.DELTA.C and the resonance frequency in the above state of the diode by f+.DELTA.f (frequency variation), .DELTA.f is represented by the following equation: ##EQU2##
Because there exists the relation, .DELTA.C&lt;C, the equation (2) can be simplified by omitting the power terms of .DELTA.C/C as follows: ##EQU3##
Thus, the sensitivity S.sub.1 is given by EQU S.sub.1 =.DELTA.f/f=-{.DELTA.C/(2C)} (3)
On the contrary, under the condition that the switching diode 24 is conducting, the whole resonance capacitance when the variable capacitance diode 13 has the minimum value of capacitance becomes identical to the sum of the above C and Ca (capacitance of the condenser 23), so that the resonance frequency f.sub.1 is represented by the following equation: ##EQU4##
Similarly, because the whole resonance capacitance when the variable capacitance diode 13 has the maximum value is C+Ca+.DELTA.C and the resonance frequency in the above state is f.sub.1 +.DELTA.f.sub.1 (frequency variation), .DELTA.f.sub.1 is given by ##EQU5##
Therefore, the sensitivity S.sub.2 is represented by the following equation: EQU S.sub.2 =.DELTA.f.sub.1 /f.sub.1 =-[.DELTA.C/{2(C+Ca)}] (6)
Accordingly, comparison of the sensitivity S.sub.1 with S.sub.2 represented respectively by the equations (3) and (6) results in the following relation: EQU .vertline.S.sub.1 .vertline.&gt;.vertline.S.sub.2 .vertline. (7)
As is appreciated from the foregoing description, the sensitivity of the VCO becomes high if the VCO 8 is switched to a high oscillation frequency band, whereas it becomes small if the same is switched to a low oscillation frequency band.